Effect of Municipal Solid Waste Incineration Fly Ash Addition on the Iron Ore Sintering Process, Mineral Phase and Metallurgical Properties of Iron Ore Sinter

Abstract

Municipal solid waste incineration (MSWI) fly ash is well known as hazardous waste, for promoting its harmless treatment and resource utilization, a new process was proposed by utilizing the iron ore sintering process employed massively in iron and steel industry. In this article, the feasibility of this process was estimated from the point of the effects of MSWI fly ash addition on the iron ore sintering process, sinter mineral phase structure and metallurgical properties through the sintering pot experiments, the reduction and melt-dropping experiments at different temperature. Results show that, for the sintering process, the flame front speed, the sinter productivity, the drum index and the yield of sinter decrease with the increasing of MSWI fly ash addition; for the sinter metallurgical properties, with the increasing of MSWI fly ash addition, the lower temperature reduction property (RDI) is improved obviously, the soft-dropping property is worsened; but the effect of MSWI fly ash addition on the middle temperature reduction property (RI) is not obvious; for the sinter mineral phase, the phase fraction of the silico-ferrite of calcium and aluminum (SFCA) and the calcium silicate (CS) both increases with the increasing of MSWI fly ash addition. The complex of oxides, silicates and chlorides with lower melting point in the MSWI fly ash change the formation of sinter mineral phase and its composition essentially, which further effect the sintering process and the metallurgical properties.

1. Introduction

Municipal solid waste incineration (MSWI) fly ash is the residue collected in the gas cleaning unit or the waste heat recovery and utilization system of incineration plants. In MSWI fly ash, the major elements (>10000 mg/kg) are Ca, Si, Mg, Fe, Al, K, Pb, Zn, O, S, and Cl as oxides, silicates, phosphates, chlorides and sulfates; the minor elements (1000–10000 mg/kg) are Ti, Mn, Ba, Sn, and Cu; the trace elements (<1000 mg/kg) are Hg, Cd, Sb, Cr, Sr, Ni, As, V, Ag, Co, Mo and Se; in addition, organic compounds are usually present in the residues such as polycyclic hydrocarbons (PAH), chlorobenzenes (CB), polychlorinated biphenyls (PCB), and polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs).¹⁾ The heavy metals such as Cd and Pb can easily leach out and contaminate soil and groundwater,^2,3) posing high potential risks to environment and human health. PCDD/Fs is also categorized as extremely toxic substance for its bad health effects in humans and animals.^4,5) For this, MSWI fly ash has been classified as hazardous waste.

For the superiority in reducing the mass and volume of municipal solid waste (MSW) and producing heat to generate electricity or steam, the incineration as the MSWs disposal method developed rapidly.⁶⁾ In some countries such as Japan, Denmark, Sweden, etc., the incineration has become to be one of the dominant treatments for MSW. In China, with the urbanization process, especially in advanced cities like Beijing, Shanghai, Shenzhen, by the end of 2015, the number of municipal solid waste incinerators is about 220.⁷⁾ For example in Shanghai in 2015, about 40% (2500 thousand tons) of MSW was treated by the incineration method, and about 100 thousand tons of MSWI fly ash was produced.⁷⁾ In order to prevent the risk of environmental pollution, MSWI fly ash must be treated innocuously before filling in the land or resource utilization. With the increasing of MSWI fly ash, there exist serious problems and challenges with how to treat it.

Different methods have been developed to process the MSWI fly ash in order to immobilize, remove or destroy harmful components and make the ash utilizable or safe to dispose of. Reviewing the main treatment methods, which can be divided into three groups:^1,8,9,10) (1) physical or chemical separation process^{11,12,13,14,15)} such as washing, chemical precipitation, chemical extraction etc.; (2) stabilization and solidification process^16,17,18,19) such as solidification/stabilization with hydraulic binders, chemical stabilization and ageing/weathering; (3) thermal treatment process^{20,21,22,23,24,25,26,27,28,29,30,31)} such as sintering, vitrification, melting or fusion, which is considered as the best methods for the harmful metals separation or stabilization, harmful organic compounds decomposition, and the application of the products.

For the sintering, some studies^{21,22,23,26,27,28,29,30,31)} have been performed on MSWI fly ash in the temperature range of 700–1200°C to produce various types of products, such as concrete aggregates, ceramic tiles, etc. During sintering, part of the harmful heavy metals such as Cd, Pb, and Hg are vaporized and almost all the organic pollutants are decomposed, which decrease the leachability of harmful components from the end product. Nevertheless, one or more pretreatment steps such as washing, sieving, ball milling, drying or compaction/pelletizing are often required before the sintering step to guarantee the quality of sintered products.^32,33) For the vitrification,^20,23) the MSWI fly ash is melted with additives or other waste solids during 1100–1500°C and subsequently cooled to form an amorphous, homogenous single glass phase. Although the vitrification is most effective in reducing the toxicity of Cr, Cd, and Pb etc., the raw materials such as sand (SiO₂), feldspar (NaAlSi₃O₈, KAlSi₃O₈) and limestone (CaCO₃), etc. must be added to form glass for harmful metals bonding or encapsulation. For the melting or fusion process,²⁵⁾ which is similar to vitrification, but no additives are used and the end product is usually a heterogeneous slag mixture, consisting of glassy material and crystalline phases. Moreover, reviewing all the thermal treatment methods, excess energy such as gas, oil, coal or electricity are needed, which lead to the disadvantage in economy, especially in a separate treatment plant. Therefore, although the thermal treatment methods are feasible in technology, there still exist economic and social issues as the significant impediments to large scale adoption in dealing with MSWI fly ash.

To over the disadvantage of excess energy consumption, the most promising treatment method of MSWI fly ash is utilizing the existing high temperature industry process with sufficient processing capacity. In the present paper, we propose a new method of utilizing the iron ore sintering process by adding MSWI fly ash into raw materials directly with no pretreatment. Except for the advantages of remove or destroy harmful components as high temperature process, some other advantages can be predicted as follows: (1) Under the condition of suitable addition quantity, the elements containing in MSWI fly ash are diluted sufficiently and come into the iron making process, so no subsequent deposing process is needed; (2) Fully utilizing the flue gas cleaning system to control the emissions of SO₂ and PCDD/Fs potential increased by MSWI fly ash addition; (3) Promoting the coordinated development of steel plant and its location city, especially for the steel plant which locate in the advanced city. For Baosteel of Shanghai, if the mass addition ratio is 1%, almost 200 thousand tons of MSWI fly ash can be consumed annually, about two times of MSWI fly ash output of Shanghai in 2015. To evaluate the feasibility of iron ore sinter manufacturing with co-combustion of MSWI fly ash, the effects of MSWI fly ash addition on the sintering process, the sinter strength and the metallurgical properties must be identified firstly.

2. Materials and Methods

2.1. Materials

Raw materials including iron-bearing materials, fluxes and fuels were used for the laboratory sintering studies, which taken from stock yard as well as beneficiation plant classifier product. Sinter return fines (particle size less than 5 mm) and iron ore fines (particle size less than 10 mm) are used as iron-bearing materials. Except domestic iron ore fines, the others imported from Australia, Brazil and South Africa. Limestone, dolomite and calcined lime are used as fluxes with particle size less than 3 mm. Coke breeze was used as fuel with particle size less than 3 mm. Table 1 shows the chemical compositions of raw materials, the contents of elements of Na, K, Cl, and heavy metals such as Cr, Cd and Pb etc. are all less than 0.01% which are not list in Table 1. In the experiments, on the condition of no MSWI fly ash addition, the controlling target of total ferrum content and basicity [wt(CaO)/wt(SiO₂)] is 58.0% and 1.8 respectively, according to this, the calculation results of material ratios are also listed in Table 1. On the condition of MSWI fly ash addition, it is merely overlaid on the raw materials without restrictions of total ferrum content and basicity, and the normalized addition ratio of MSWI fly ash is 1%, 2%, 3% and 5% respectively. MSWI fly ash used in the experiments was taken from one municipal solid waste incinerator in Shanghai of China and the chemical compositions are shown in Table 2.

Table 1. Compositions and ratios of iron bearing materials, fluxes and fuel used in sintering experiments (mass,%).

Raw materials	Fe	CaO	SiO2	MgO	Al2O3	H2O	S	P	LOI	Ratio
Domestic ore	69.17	0.27	3.19	0.38	0.36	8	0.023	0.050	−3.03	40.5
Australia ore	58.41	0.03	6.16	0.14	2.32	7	0.025	0.044	7.36	10.0
Brazil ore	63.93	0.01	5.09	0.13	3.28	8	0.007	0.046	2.05	10.0
South Africa ore	62.81	0.34	5.91	0.12	1.87	7	0.010	0.050	1.57	5.0
Return sinter fines	64.04	1.03	5.39	1.18	1.98	2	0.022	0.028	0.29	18.0
Dolomite	0.00	1.40	3.31	43.52	0.00	2	˂0.010	˂0.010	48.70	1.2
Limestone	0.00	50.89	2.23	2.42	0.18	2	0.050	0.010	42.30	7.8
Calcined lime	0.00	82.11	2.99	2.77	0.47	1	0.025	0.010	8.16	4.0
Coke breeze	0.00	0.00	8.00	0.29	0.00	6	0.820	0.170	72.00	3.5

Table 2. Compositions of MSWI fly ash used in the sintering experiments (mass,%).

Fe	CaO	SiO2	MgO	Al2O3	Na2O	K2O	Cr2O3	S	P	Cl	Zn	Pb	Sn	Cu
0.99	36.58	11.71	2.55	3.95	0.07	0.07	0.01	1.16	0.075	12.42	0.23	0.05	0.06	0.02

2.2. Methods

2.2.1. Sintering

The experiments were conducted via a sintering pot system, which equipped with igniter, thermocouple, and dust precipitator followed by a fan and a stack. After weighting according to the ratio in Table 1, raw materials and MSWI fly ash were mixed for the first time in a cylindrical mixer (600 mm in diameter, 1200 mm in length) for 4 minutes and granulated by the second mixing with sprayed water (moisture content within 7.5% to 8.0% in weight) for 4 minutes to produce granules before they were charged into a sinter pot (300 mm in diameter, 690 mm in height) to compose the packed bed on 4 Kg hearth layer of 25–45 mm sinter particles. After 0.2 Kg coke breeze were spread uniformly onto the surface of the packed bed for better ignition, the top layer was ignited with the mixture of liquefied petroleum gas and compressed air for a period of 2 minutes with temperature of about 1100°C. During ignition, the suction pressure was maintained 8 KPa, after ignition, the suction pressure was increased to 17.5 KPa and maintained till the completion of sintering when the temperature of exhaust gas began decreasing from the highest point. When the temperature of exhaust gas was lower than 100°C, the sinter cake (weighed as G₁) was dislodged out of the pot and then dropped of 2 m height for 4 times. After dropping, it was sieved in fractions of > 40 mm, 40–25 mm, 25–16 mm, 16–10 mm, 10–5 mm and <5 mm, all the fractions were weighed and the weight of the fractions which particle size larger than 10 mm was recorded as G₂. After that, 15 Kg sinter was selected which particle size was larger than 10 mm according to their weight fractions for the drum test (25 rpm, 8 min), then sieved the fraction particle size larger than 6.3 mm and weighed as G₃. The indices of flame front speed, sinter productivity, sinter yield and drum index are calculated as Eqs. (1), (2), (3), (4):   

$$\mathrm{F}\mathrm{l}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{f}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{e}\mathrm{d}:\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }V=\mathrm{H}/t,\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{m}\cdot \mathrm{m}\mathrm{i}{\mathrm{n}}^{-1}$$(1)

  

$$\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}:\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{N}=G_2/\left(\mathrm{A}t\right),\mathrm{T}\mathrm{o}\mathrm{n}\cdot {\mathrm{m}}^{-2}\cdot {\mathrm{h}}^{-1}\cdot$$(2)

  

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}:\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }{n}_{\mathrm{n}\mathrm{p}}=G_2/G_1\times 100,\mathrm{\hspace{0.17em} }\%$$(3)

  

$$\mathrm{D}\mathrm{r}\mathrm{u}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}:\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\delta =G_3/15\times 100,\mathrm{\hspace{0.17em} }\%$$(4)

where, H is the effective height of sintering pot (=690 mm); A is the cross-section area of the sinter pot (=0.07 m²); t is the time from ignition to sintering terminal (time of the temperature of exhaust gas began decreasing from the highest point), minute in Eq. (1), and hour in Eq. (2).

2.2.2. Testing of Metallurgical Properties

All the sinter products with different MSWI fly ash addition were tested for the reduction degradation index (RDI_+3.15) and the reduction index (RI) according to international standard of ISO4696 and ISO4695 respectively. The sinter products of 0%, 1% and 3% MSWI fly ash addition were measured for the melt-dropping property via load-reduction experiment system. In these experiments, 0.5 Kg of sinter products within 10.0–12.5 mm diameter were charged into a graphite crucible of 75 mm diameter and with dropping holes of 10 mm diameter at the bottom. Before sinter charging, coke breeze was spread uniformly on the bottom of the crucible with 30 mm height, and after this charging, coke breeze was spread uniformly on the surface of the sinter bed with 15 mm height. The experiment parameters including load, gases and their flow rate, temperature controlling are listed in the Table 3. During experiments, the softening start (sinter bed shrinks 10%) temperature T₁₀, the softening terminal (sinter bed shrinks 40%) temperature T₄₀, the melting start (pressure difference between the inlet and the outlet of the reduction gas increasing sharply) temperature T_s and the dropping temperature T_d are recorded.

Table 3. Parameters of melt-dropping experiments on sinter.

Time, min	0–40	40–90	90–130	130 to end
Load, kg/cm2	0.5	0.5	2.0	2.0
Gas flow rate, L/min	N2: 3	N2: 9/CO: 3.9/CO2: 2.1	N2: 10.5/CO: 4.5	N2: 10.5/CO: 4.5
Heating rate,°C/min	l0 (≤400°C)	l0 (400–900°C)	3 (900–1020°C)	5 (1020°C to dropping)

2.2.3. Chemical and Mineral Structural Analysis

Sinter products of all experiments were collected for the chemical analysis via ICP–AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). For the mineral phase structure analysis, the sinter particles of about 20 mm size were mounted in epoxy resin and polished with silicon carbide paper up to 1000 grit, eventually, samples were polished with an automatic grinder polisher under a fixed load to 0.5 micron. After this, the mineral phases were observed with Scanning Electron Microscope (Shimadzu Corp. Japan, SSX-550^TM) equipped with an Energy-dispersive X-ray Spectrometer (EDS), and 49 continuous photos were taken for the mineral phase fraction estimation by using the image analysis software of Image-Pro Plus v 6.0.

3. Results and Discussions

3.1. Effect of MSWI Fly Ash Addition on Sinter Mineral Phase

The SEM images of sinter microstructures are shown in Figs. 1(a) to 1(c) in terms of MSWI fly ash addition ratio of 0%, 1% and 3%, and the variations of phase compositions (average value of 5 points of SEM-EDS analyses) are shown in Figs. 2(I) to 2(III) correspondingly. There are three types of phase in the microstructures: the first type is the original iron ore phase (white, marked with F, for the reason of EDS analyses results, shows slightly deviation from the average content of irons fines), the second is the silico-ferrite of calcium and aluminum (dark grey, marked with SFCA) surrounding or connecting with the phase F, the third is the slag phase of calcium silicate containing some content of ferrite (black, marked with CS). In addition, there are some fraction of pores in the sinters which are almost all associated with CS phases, it is difficult to distinguish them from each other, so during phase area estimation, they are treated as one (marked with CS+P).

Fig. 1.

SEM images of sinter microstructures, MSWI fly ash addition ratio 0% (a), 1% (b) and 3% (c).

Fig. 2.

Variations of mineral phase compositions with MSWI fly ash addition, (I) Phase CS, (II) Phase SFCA, (III) Phase F.

Figure 3 shows the variation of phase area fraction with the MSWI fly ash addition ratio. The phase fraction of F decreases from 56.2% to 54.2% and 47.6% with the MSWI fly ash addition ratio increases from 0% to 1% and 2% respectively. On the contrary, the phase fraction of SFCA increases from 31.1% to 32.5% and 34.2%, and fraction of CS+P increases from 12.7% to 13.3 and 18.2% respectively. For the iron ore sinter, the pores basically form from the inheritance of gap not filled in the packed bed, burning of coke breezes, and shrink by the formation of liquid melt. As the MSWI fly ash is a typical micro powder with particle size of less than 100 μm, which is in favor of filling in the gaps among larger particles of the raw materials, this will lead to the decreasing of the volume of gap not filled. Moreover, the volume fraction of coke breeze would decrease with the MSWI fly ash addition (overlaid on the raw materials). Such consideration suggests that the observed increase of CS+P should come from the increase of CS volume itself rather than the increase in porosity.

Fig. 3.

Variation of mineral phase fraction of sinter with MSWI fly ash addition.

Figure 1 also shows morphological change of SFCA phase. With the increasing of MSWI fly ash addition, the crystal structure of the SFCA phase changes from acicular morphology to columnar morphology. On the condition of no MSWI fly ash addition as shown in the Fig. 1(a), the acicular SFCA phases crisscross each other and form a grid tightly, with the increasing of MSWI fly ash addition, especially on the condition of MSWI fly ash addition ratio of 3% as shown in the Fig. 1(c), the columnar SFCA phase is segregated by the CS phase obviously.

Contrasting the compositions of fluxes in Table 1 and the compositions of MSWI fly ash in Table 2, it shows that, the contents of SiO₂, Al₂O₃ and Cl in the MSWI fly ash are all higher than the raw materials. The phase transformation is mostly induced by the changes of chemical composition with the MSWI fly ash addition.

The MSWI fly ash has lower melting point³⁴⁾ and smaller particle size than the raw materials. This³⁵⁾ will promote the formation of primary melt early via reaction with other raw materials or melting under lower temperature. Such behavior will accelerate the assimilation of iron ore particle to form more SFCA phase.³⁶⁾ As for phase compositions of SFCA Fig. 2(II) shows that the content of Fe (73.46%, 72.85% and 71.75%) decreases and the content of Si and Al increase with the increasing of MSWI fly ash addition, which is in accordance with the report of Mumme et al. that the transformation from high-Fe, low-Si, low-Al form (acicular SFCA) to the low-Fe form (columnar SFCA).³⁷⁾ On the other hand, the primary melt with lower melting point will accelerate the assimilation of dolomite to form more CS phase, this also results in the increasing of content Ca with the increasing of MSWI fly ash addition as shown in Fig. 2(I). Figure 2(I) also shows the increasing of Al and Cl contents in the CS phase, which means that part of MSWI fly ash moves to CS phase.

In conclusion MSWI fly ash addition increased the amount of SFCA and CS both (Fig. 3) in accordance with the change in their chemical composition (Fig. 2).

3.2. Effect of MSWI Fly Ash Addition on the Sintering Process and Sinter Strength

Figure 4 shows that, with the increasing of MSWI fly ash addition, the flame front speed (denotes the vertical sintering speed) decreases observably. In sintering process, on the same condition of height of packed bed and the suction pressure, the flame front speed mainly depends on the sinter bed permeability and it mainly depends on the bed structure and the chemical compositions of raw material.^35,38) In these experiments, the addition of MSWI fly ash will induce the increasing of fraction of smaller particles which favor filling in the gaps among larger particles, this leads to a denser packed bed; on the other hand, as the discussion in chapter 3.1, the addition of MSWI fly ash will accelerate the formation of the primary melt and increases the melt quantity which would enlarge the thickness of the combustion zone. These two aspects all decrease the bed permeability, resulting in the longer sintering time and the lower flame front speed. Sinter productivity (in Fig. 4) shows the similar trend with the flame front speed.

Fig. 4.

Variation of the flame front speed and the productivity of sintering pot with the MSWI fly ash addition ratio.

Figure 5 shows the variation of the sinter yield and strength with the MSWI fly ash addition. The sinter yield, on the condition of MSWI fly ash addition ratio not more than 2%, does not change obviously, but when MSWI fly ash addition ratio larger than 2%, the sinter yield sharply decreases from 51.83% (average value of MSWI fly ash addition ratio no more than 2%) to 38.23% and 34.47% of MSWI fly ash addition ratio of 3% and 5% respectively. The sinter strength (Drum index) also sharply decreases from 69.97% (MSWI fly ash addition ratio 2%) to 53.47% and 54.80% (MSWI fly ash addition ratio 3% and 5%) respectively. Figure 1(c) shows that, on the condition of MSWI fly ash addition ratio of 3%, the SFCA phase exists as columnar morphology and is segregated by the CS phase obviously. As the key bonding materials, the change of SFCA morphology may weaken the phase bonding strength which induces the yield and drum index decreasing.

Fig. 5.

Variation of yield, drum index, RDI_+3.15 and RI of sinter with the MSWI fly ash addition ratio.

All the results show the negative effects of MSWI fly ash addition on the sintering process and sinter strength, but it is acceptable up to 2% as far as the sinter strength and yield maintain. The limit of dosage is a recommendation based on this research though the suitable addition ratio of MSWI fly ash should be decided after further confirmation considering its effect on the emission of some pollutants such as SO₂ and PCDD/Fs.

3.3. Effect of MSWI Fly Ash Addition on RDI and RI

Figure 5 also shows the variation of reduction degradation index and reduction degree with the MSWI fly ash addition ratio. MSWI fly ash addition improves RDI obviously. When the MSWI fly ash addition ratio increases from 0% to 1%, the reduction degradation index RDI_+3.15 increases from 59.61% to 92.52% drastically. When the MSWI fly ash addition ratio exceeds 1%, the reduction degradation index RDI_+3.15 increases gently from 92.52% to more than 97% of 5% MSWI fly ash addition. RDI is induced by the stress with phase transition of hematite to magnetite. If the hematite is coated with other phase such as CS, it will impede the diffusion of CO gas to inner cores and the subsequent reduction reaction. As discussion in chapter 3.1, the phase fraction of SFCA and CS all increase with the increasing of MSWI fly ash addition obviously, which decreases the reaction area between hematite and CO, and then decreases the phase transition fraction of hematite to magnetite and less volume change, so decreases the degradation ratio.

RI changes not obviously with the increasing addition of MSWI fly ash and fluctuates between 77.72% and 88.33%. The behavior may reflect the complex influences as follows: On the condition of sinter basicity near 1.8, CS phase mainly exist as 2CaO·SiO₂ (C₂S). During the heating process up to RI testing temperature of 900°C, crystalline transition (e.g. γ-C₂S to α’-C₂S near 725°C, not occurs below the RDI testing temperature of 500°C) will lead to volume expansion and degradation of coated CS phase, naturally, the impediment of CO penetration will be alleviated. Moreover, at this temperature, SFCA can be easy reduced which provides more diffusion channel for CO.

3.4. Effect of MSWI Fly Ash Addition on the Melt-dropping Property of Sinter

Figure 6 shows the variation of softening and dropping temperature of sinter with the MSWI fly ash addition ratio. T₁₀ and T_d all decrease with the increasing of MSWI fly ash addition, as the temperature range of softening zone enlarge from 209.1°C to 245.4°C and 248.5°C when the MSWI fly ash addition ratio increases from 0% to 1% and 3%. For the sinter, the phase composition and its volume are both important factors affecting the softening and melting behavior. During the reduction process of sinter in blast furnace, the lower melting temperature phase formed during the sintering process will soften and melt earlier at lower temperature, so worsen the sinter melt-dropping property.

Fig. 6.

Variation of softening and dropping temperature of sinter with the MSWI fly ash addition ratio.

4. Conclusions

To propose a new method for rendering MSWI fly ash harmless by utilizing the iron ore sintering process, effects of MSWI fly ash addition on the sintering process, mineral phase, and metallurgical properties of iron ore sinter were estimated. The main conclusions are as follows:

(1) MSWI fly ash addition to the raw materials of iron ore sinter, owing to the earlier formation of molten phase and the changing of mineral phase structure for its lower melting point and ultra-fine particle size, lead to the deceasing of the flame front speed, the productivity, the sinter yield and drum index.

(2) MSWI fly ash addition improved RDI because of the increasing of CS phase, however, the softening and dropping temperature of sinter all decrease for the same reason of the increasing of CS phase with lower melting temperature. RI changes not obviously.

(3) MSWI fly ash addition is acceptable up to 2% as far as the sinter strength and yield maintain.

Acknowledgements

The authors gratefully acknowledge the support from National Key R & D Program of China (Grant No. 2017YFC0805102), National Natural Science Foundation of China (Grant No. NSFC 51374060, 51674069), and Baosteel Development Ltd.

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